This is a term I heard a lot when I was growing up because my dad, grandpa, and great grandpa were all well diggers. Many of the farmers thought it was funny. Which it is. The reference is to the old days of well diggers when they would dig the well by hand and have to be fairly deep in the ground. Once you get to 10 feet under the surface, in a normally open area (not tunnels in cities), the temperature is pretty constant at 53 degrees. Most groundwater from the central states is 53 degrees. This is also noticeable when water pipes and sewers are always buried 5 feet or more deep to keep them from freezing.